1. Technical Field
The present invention relates generally to a filter and filter element construction. In particular, the present invention relates to a filter element that includes a supporting substrate made from at least two materials with different temperatures at which they are able to soften and flow.
2. Description of the Prior Art
It is known that efforts throughout the world to improve air quality continually increase through treaties, laws and regulations which limit the amount of industrial emissions that may be released into the atmosphere. Improved and more efficient filters that minimize and reduce the release of emissions have been developed. One such filter includes a filter element with a microporous expanded polytetrafluoroethylene (ePTFE) membrane. The membrane assumes a relatively large percentage of the filtering duties when attached to a support structure, such as a substrate, by a mechanism referred to as xe2x80x9csurface filtrationxe2x80x9d.
It is known that problems can result from attaching together the membrane and the substrate. For example, the membrane may be attached to the substrate by a laminating operation that typically includes thermal bonding. In one type of thermal bonding process, fibers at an external surface of the substrate are singed to increase the area of the substrate that can contact the membrane. The membrane and singed areas of the substrate are brought into intimate contact. The singed areas of the substrate are then melted onto the membrane by the application of heat. The melted portions of the substrate flow into pores in the membrane. Once the melted portions of the substrate cool and solidify, the membrane is fixed to the substrate.
However, the material used in the substrate can melt uncontrollably and engage a relatively large surface area of the membrane. Melted portions of the substrate that solidify can block a substantial portion of the pores in the membrane and reduce the effective filtering area of the membrane. Thus, filtration efficiency is detrimentally affected due to less than maximum airflow through the filter element and a relatively large pressure drop across the filter element can result. It is also known that as the melted portions of the substrate cool to solidify and attach to the membrane, the melted portions may contract. When the melted portions of the substrate are exposed to changes in temperature, such as during thermal cycling in the environment in which the filter element operates, the melted portions may expand and contract. The membrane may not be able to match this expansion and/or contraction which can overstress the membrane causing the membrane to tear or crack adjacent the melted portions.
The use of a needled felt substrate having core-sheath copolyester/polyester (COPET/PET) fibers for thermal bonding to produce paper-like and lofty non-wovens without the use of resin bonding is known. However, the known COPET materials used are amorphous copolyesters with softening points in the range of 65xc2x0 C.-180xc2x0 C. (150xc2x0 F.-355xc2x0 F.). The anticipated continuous use operating temperatures of certain filter elements made from an ePTFE/felt substrate laminate can be in the range of about 135xc2x0 C.-150xc2x0 C. (275xc2x0 F.-300xc2x0 F.). It is critical that the bonding material does not soften to flow or melt when continuously exposed to these relatively high operating temperatures to ensure that the mechanical adhesion and dimensional stability of the laminate is accomplished and maintained throughout the service life of the laminate. It is also important that the substrate does not expand and contract an amount that the membrane cannot accommodate.
Thus, there is a need in the filtration art for an improved filter element that is durable when exposed to various environments at relatively elevated temperatures and thermally cycled.
The present invention is directed to a filter element. The filter element includes a porous membrane and a substrate made from at least two different materials. A first one of the materials has a first temperature at which the first material activates. A second one of the materials is at least partially crystalline and has a second temperature which is lower than the first temperature at which the second material activates. The porous membrane is supported by the substrate. An attachment exists between the substrate and the membrane. The attachment results from the second material of the substrate being activated at a temperature less than the first temperature to engage portions of the membrane at spaced apart locations and fix together the membrane and the substrate upon the second material cooled to a temperature less than the second temperature.
The porous membrane is microporous and made from expanded polytetrafluoroethylene. At least a portion of the substrate comprises a multiplicity of bicomponent staple fibers selected from the structures including core-sheath, side-by-side, segmented and lobed. A portion of each of the fibers is made of the first material. Another portion of each of the fibers is made of the second material.
The substrate may further include a cap and a base. The cap is located adjacent the membrane. The cap includes the second material present in a percentage of not more than fifty percent by weight or by volume. The second material is temperature stabilized at a temperature less than the second temperature. Each of the fibers has a diameter of 40 microns or less. Each of the fibers has a denier of 8 or less.